The peer-to-peer aspect of YC already scales. The hard part to scale is what we partners do.
The latter can work only by forming these new angel-firms in different startup-beds. There are only a limited number of startup-beds, and a limited number of connections that can be used by partners. When these resources are used up, the only choice is to move to another area and start again.
The former is what YC already does, within the limits of the partners. Adding new partners could increase the number of companies funded each year, but only to a limited extent. Finding a partner is hard because it needs to be someone that won't disturb the current dynamic and who will bring in new connections/resources that aren't currently represented.
Frankly, I don't see any way to truly scale what YC does. Encouraging startups to form is limited by its very nature to within bounds formed by the market. These bounds have not yet been reached, but still exist.
YC would then grow like a B-tree. In the beginning, all founders are on the same "page". As the page grows beyond its size limits, the page is split, successful founders are promoted up to the parent level, and there's more room to new founders at the leaves.
You'd need to run it by former YC-entrepeneurs, and it'll probably be a few years until their startups are self-sustaining (or bust) enough for them to take on other projects. But if they're like other entrepeneurs at all, they'd probably love to take a role in mentoring new YC applicants.